There is currently great interest in finding energy sources that are alternatives to petroleum and coal, since we are currently running out of such resources, and perhaps more importantly, because it is becoming increasingly apparent that the burning of these non-carbon-neutral energy sources may be contributing to global climate change.
One area of alternative energy production that has received significant attention is photovoltaics, the conversion of light into electricity, most commonly using silicon based solar cells. Elemental silicon (Si) is produced via the Carbothermic Process, whereby silicon dioxide (typically quartz) is chemically reduced in an electric arc furnace at temperatures in excess of 1400° C. The reducing agents used in this process are carbonaceous, typically coal, charcoal, petroleum coke and wood chips. The net chemical reaction for Si production is:SiO2+2C→Si+2CO
One major problem with the current art is that, based on the low purity of starting materials, the Si product often contains high levels of undesirable elemental impurities. Such “Metallurgical Grade” (MG) silicon is often only 98% pure, and is entirely unusable in this form for the production of solar cells. One commonly employed method for purifying this MG silicon is to convert it through a high temperature reaction to the low boiling point compound trichlorosilane (SiHCl3), purify this chemical by distillation, then back-convert the SiHCl3 to elemental silicon in the presence of hydrogen gas at temperatures of ca. 1100 to 1200° C., generating HCl gas as a by-product. This purification process is very expensive due to energy requirements, chemical handling requirements, etc. In addition, the cost of the final silicon is approximately 5 to 10 times that which is desired for photovoltaic applications. Furthermore, the purity of the final product far exceeds the requirements for solar-cell manufacture.
A second method for producing photovoltaic (PV) grade silicon is by purifying or “upgrading” MG silicon to a level acceptable for solar-cell manufacture without the need for SiHCl3 as an intermediate. However, two silicon impurities that greatly degrade the performance of solar cells, Boron (B) and Phosphorus (P), cannot readily be removed from silicon using commonly employed methods in the art of silicon purification, such as directional solidification.
Although many novel processes have been devised to selectively remove B and P from MG silicon, these methods are expensive, since the MG silicon starting material must be re-melted to above 1400° C. prior to purification, increasing the final silicon cost due factors such as energy, labor, capital equipment and yield losses.
It would be highly desirable and advantageous to either synthesize PV grade Si directly from high-purity starting materials, or use such high-purity starting materials to prepare Si that requires a minimum of post-synthesis purification prior to use in the manufacture of solar-cells. Attempts have been made to prepare PV grade silicon by beginning with high-purity natural quartz and carbon sources. However, the disadvantages of this method include the need to mine the quartz and crush it into smaller pieces prior to reduction to elemental silicon. These steps further increase the cost of the starting material and are a potential source of further impurity introduction. Also, the geological source of the quartz material may be a significant distance from the location of final silicon manufacture, further increasing costs due to transportation. Furthermore, quartz, being a crystalline form of SiO2, is a known cause of the disease silicosis, which results when dust from such materials is inhaled.
There exist methods for isolating biogenic silica, that is, SiO2 that is developed or assimilated in the cell structures of living organisms, from plants or parts of plants, such as rice hulls. These isolation methods typically involve burning the rice hulls and recovering the siliceous ashes for further use (Pitt, U.S. Pat. No. 3,889,608, and U.S. Pat. No. 3,959,007; Mehta, U.S. Pat. No. 4,105,459). However, this method of biogenic silica recovery tends to fuse or incorporate undesirable inorganic elements such as B and P into the silica during this high temperature processing, as well as reduce surface area of the silica due to pore collapse and closure. Furthermore, in many embodiments the silica derived from rice hulls is further refined by dissolving the silica in strong base, then precipitating it out of solution by the addition of acid (Stephens et. al, U.S. Pat. No. 6,375,735; U.S. Pat. No. 6,638,354; and U.S. Patent Application Publication US 2003/0097966; Kang, U.S. Pat. No. 6,843,974; Connor and Rieber, U.S. Pat. No. 5,078,795, U.S. Pat. No. 5,008,021; Rieber et al., U.S. Pat. No. 5,833,940; Shipley, U.S. Pat. No. 6,406,678).
Isolation of biogenic silica during the strong acid hydrolysis of rice straw has also been reported (Farone and Cuzens, U.S. Pat. No. 5,782,982). Sugars derived from this process are separated and purified prior to metabolic conversion into ethanol. The silica isolated from this strong acid hydrolysis process is dissolved using strong base, the supernatant liquid isolated and silica re-precipitated by lowering the pH of the supernatant liquid using acid. A major disadvantage of this methodology is the need to isolate the silica using strong bases and acids, which is very expensive due to the requirements of chemical storage, handling and disposal.
Biogenic silica derived from rice hulls has been used as a starting material for the preparation of silicon (Amick, U.S. Pat. No. 4,214,920). In these methods, rice hulls were typically first washed with water and/or dilute hydrochloric acid prior to being heated in either an inert or an oxygen atmosphere between 600° C. and 800° C. to carbonize or remove organic components, yielding a siliceous product that was then used to produce silicon. The chief disadvantage of these methodologies is the elimination or carbonization of organic materials such as cellulose from the rice hulls via simple burning, instead of isolating these materials and using them as feedstocks to create other economically valuable organic co-products. In addition, rice hulls, being very low density, are very expensive to transport, and any process utilizing rice hulls is consequently practically limited to a location close to rice-growing agricultural areas.
A second area of alternative energy production that has received significant attention is ethanol generation employing yeast fermentation of high-starch seeds from plants such as corn and wheat. However, there are significant problems with this method of ethanol production. For instance, this technology uses “food crops” and therefore competes with human and farm animal food supplies. Growing and harvesting of these crops is also “energy intensive” in terms of land preparation, the use of fertilizers and pesticides, irrigation requirements and the energy required to harvest and transport the crop materials to central processing locations. It has been estimated that it can take as much as 30 to 50 gallons of petroleum to produce one acre of corn for ethanol generation. In addition, there are concerns that agriculture techniques used to grow such crops may have long-term destructive effects such as soil erosion and water table contamination. Furthermore, there are limited ranges of suitable land where such crops can be cultivated. Also, the current art of utilizing grain crops such as corn to produce ethanol fuel is only economically viable through the generation of co-products, such as residual materials that may be sold for uses such as animal feed.
An alternative method of creating ethanol fuel from biomass involves the use of cellulose as a feedstock. There are currently two major methods of producing ethanol from such feedstocks: 1) a “Two-Step” process whereby the cellulose is broken down either enzymatically or chemically to glucose or cellobiose, followed by fermentation to ethanol by yeasts, and 2) a “One-Step” process whereby the cellulose is metabolized directly to ethanol under anaerobic conditions by cellulolytic thermophilic bacteria. One significant problem with current cellulose ethanol production is that many feedstocks (e.g., wood sources, grasses, etc.) contain a large percentage (as high as 30%) of lignin, a dense material which encapsulates the cellulose constituents, severely inhibiting or preventing access to this cellulose for breakdown into sugars by hydrolysis using enzymes or ethanol-producing microbes. Consequently, extensive pre-processing of cellulose-containing feedstocks such as wood is required to make the cellulose readily accessible to hydrolysis. Such pre-processing methods include mechanically reducing the size of the feedstock (e.g., converting wood to sawdust), harsh chemical treatments to separate the lignin from the cellulose, etc. These methods are highly energy intensive, can create significant amounts of chemical waste, and produce large quantities of lignin-based by-products, which have limited industrial value and are toxic.
There is accordingly a need for additional methods for producing silica and ethanol from plant material, which overcome at least some of the disadvantages associated with prior art methods.